1. Field of the Invention
This invention generally relates to radio communication devices, and more particularly, to a transmission technology suitable for a high frequency range as high as at least 30 GHz.
2. Description of the Related Art
Recently, there has been considerable activity in the research and development of communication technology using the submillimeter-wave frequency range or millimeter-wave frequency range as high as 30 GHz or more. In the above-mentioned high frequency range, it is difficult to generate the stabilized local oscillation frequency. The high-quality radio communication cannot be realized unless the stabilized local oscillation frequency is available on both the transmitter and receiver.
Japanese Patent Application Publication No. 2001-53640 (hereinafter referred to as Document 1) describes a technique for transmitting an unmodulated carrier having a local oscillation frequency together with a modulated radio signal. On the receiver, the received unmodulated carrier is used as the local oscillation frequency to demodulate the modulated radio signal. It is not necessary to provide a high-accuracy local oscillator on the receiver. It is thus possible to simplify the structure of the receiver. In the case where the unmodulated carrier and the modulated radio carrier are affected by the same environmental factor such as temperature change, the affect of the environmental factor (fluctuations due to the temperature change) can be cancelled by demodulating with the received unmodulated carrier, and an excellent communication quality can be thus provided.
The invention disclosed in Document 1, however, employs a local oscillator that generates the oscillation frequency identical to that of the unmodulated carrier to be output from an antenna. There is a problem in that the stable modulated radio carrier cannot be generated. In order to obtain the local oscillator capable of stably generating the oscillation frequency of at least 30 GHz, there are still drawbacks to be solved even with the use of current technology. In particular, an extremely advanced technology and a considerable cost are required for producing the local oscillator having a frequency range as high as 60 GHz in order to realize the millimeter-wave radio communication with a frequency range of 60 GHz, which is considered attractive these days.